1. Field of the Invention
The present invention relates to an improvement of a multiplex system used in a hierarchical data transmission system in which a plurality of signals output by multiplexers are further multiplexed and transmitted to a transmission line, especially an optical transmission line.
2. Description of the Related Art
A hierarchical transmission system is known in which, for example, 9-channel data trains, each having a transmission rate of 45 Mb/s, are multiplexed by low group multiplexers to obtain a multiplexed data train of 405 Mb/s and, for example, two of these multiplexed data trains are further multiplexed by a high group multiplexer to obtain a multiplexed data train of 810 Mb/s, and this high group multiplexed data train is electro-optic converted by an optical interface circuit and transmitted to an optical transmission line, whereby a large amount of data transmission is performed. In this system, the output signals of the low group multiplexers are scrambled at the highest speed now possible, whereby the ratio of "1" and "0" of the output signal becomes almost equal.
In such a system, when one of the low group multiplexers becomes unavailable through a fault occurrence, the unavailable multiplexer outputs "0" or "1" continuously. Accordingly, the ratio of "0" and "1" of the output signal does not become equal.
However, when the unavailable low group multiplexer outputs, for example, "0" continuously, if the transmission rate of the system is very high and the number of low group multiplexers is small, for example, two, it is difficult to extract a timing clock from a received signal in the receiver side, therefore normal reception becomes impossible. On the other hand, when the unavailable low group multiplexer outputs "1" continuously, if this system is used in the optical transmission system, the lighting time of a light emission element, for example a semiconductor laser, becomes long in comparison with the case in which a fault does not occur, and accordingly, the lifetime of the light emission element is shortened.